The fundamental principle behind loudspeaker operation is the controlled displacement of air in accordance with an applied electrical voltage signal. Usually, a membrane or diaphragm of some sort is the actual mechanical interface with the surrounding air, which oscillates in tandem with the variations in the electrical signal. If these oscillations have frequency content between approximately 20 and 20,000 cycles per second (Hz), then the resulting air pressure waves are audible to the healthy human ear, being perceived as sound.
In the audio system art, a common type of loudspeaker is the electrodynamic loudspeaker, which comprises a current-carrying coil placed in a gap, across which is applied a magnetic field. As the electric potential applied across the coil is varied, a corresponding force is exerted on the coil, causing the coil to move back and forth along its axis, perpendicularly to the magnetic field. The coil is attached to a diaphragm which displaces air as the coil oscillates.
For a given oscillatory voltage waveform applied to the coil, the oscillations will produce a certain displacement of the coil relative to the rest position of the coil, known as the "excursion" of the loudspeaker. If the applied voltage is of sufficient amplitude at a given frequency, the magnetomotive force will be large enough to cause the coil to partially exit the magnetic field. That is to say, the number of coil windings immersed in the magnetic field becomes a function of time, altering both the magnetomotive force and the loudspeaker impedance. The excursion, being directly related to the magnetomotive force for a sinusoidal voltage at a given frequency, no longer varies in accordance with the voltage as when the coil is fully immersed in the gap. Consequently, the sound produced by the loudspeaker will be distorted.
The notions of excursion and of the influence of excessive excursion on signal distortion are applicable to virtually all types of loudspeakers currently in use. Irrespective of the type of loudspeaker, therefore, a high degree of audio fidelity is achievable only if distortion is kept to a minimum. As it has been found that a primary cause of distortion is excessive excursion, it follows that it is crucial to keep the loudspeaker excursion to within acceptable limits. In the case of an electrodynamic loudspeaker, this ensures that the coil will remain entirely set in the magnetic field.
In the prior art, it is known to limit the excursion of an electrodynamic loudspeaker by adjusting the suspension mechanism so as to physically prevent the coil from leaving the magnetic field. This often leads to undesirable acoustic effects which can be as severe as audible thuds.
A more satisfactory prior art solution is to limit the voltage signal applied to the loudspeaker by inserting a so-called compressor in the audio path prior to sound reproduction. If the total energy of the audio signal at a given time exceeds a threshold value, then the compressor attenuates the voltage level of the audio signal; otherwise, the compressor does not affect the voltage level of the audio signal. The threshold value is chosen to limit loudspeaker excursion to acceptable values which do not cause distortion. Compressors of this nature are well known and used in the audio system art.
However, the use of compressors such as those found in prior art teachings does not recognize the importance of considering the behaviour of loudspeaker excursion as a function of frequency. For instance, it has been observed that for an acoustic signal level of a given amplitude, loudspeaker excursion is greater when the signal has energy clustered at lower frequencies, assuming that the signal is in the speaker's pass band. The precise relationship will depend on the mechanical, electrical and acoustical details of the loudspeaker, but excursion versus frequency generally has a low-pass character.
In a typical telecommunications scenario, if the audio signal represents human voice, then the short-term energy content of the signal will be concentrated in a region of the frequency spectrum that depends on the phoneme being uttered by the talker. If the energy is mostly to be found at lower frequencies, such as in the pronunciation of vowels, then the gain of the audio signal should be limited to prevent the onset of distortion due to loudspeaker excursion.
On the other hand, the energy may be clustered around higher frequencies, such as during articulation of fricative consonants, in which case even an increase in gain would not cause the loudspeaker to introduce distortion. However, as prior art compressors limit the gain of the audio signal solely based on total energy, and not on its distribution across the frequency spectrum, the loudspeaker may find itself reproducing signals that may have undergone unnecessary limiting. Prior art systems using audio compression are thus inefficient and do not use the entire audio system loudness headroom when dealing with signals having varying short-term frequency content.